With reference to 'Ecosystem', which of the following statements is/are correct ? 1. Green plants in terrestrial ecosystem capture about 1% of energy of the sunlight that falls on their leaves. 2. Around 10% of organic matter reaches to the next higher level of consumers. Select the correct answer from the code given below : Code :
- (a)Only 2
- (b)Both 1 and 2
- (c)Neither 1 nor 2
- (d)Only 1
Correct — B, Both 1 and 2. Both statements are lifted almost word for word from the standard school treatment of energy flow. Statement 1 is the NCERT formulation: of all the sunlight that falls on the leaves of green plants in a terrestrial ecosystem, only about one per cent is captured and converted into food energy — the rest is reflected, passes through, warms the leaf, or falls outside the wavelengths photosynthesis can use. Statement 2 is Lindeman's ten per cent law: of the organic matter and energy available at one trophic level, only about ten per cent is built into the body of the next level and so becomes available to the consumers above; the other ninety per cent is burnt off in respiration, lost as heat, or never assimilated. Since both propositions are correct as stated, the answer is (b).
- (a)Only 2 — This rejects the one per cent figure, usually because the candidate remembers a higher number such as 'two to ten per cent'. That higher figure is measured against photosynthetically active radiation only — a much smaller denominator. Against total incident sunlight the capture is about one per cent, which is precisely what statement 1 says.
- (c)Neither 1 nor 2 — Both statements are standard, correct textbook propositions about energy flow. Rejecting both would mean rejecting the ten per cent law itself, which is one of the most settled generalisations in ecology.
- (d)Only 1 — This accepts the one per cent capture but rejects the ten per cent transfer, and the ten per cent transfer is the better-established of the two. Raymond Lindeman set it out in 1942, and it is why food chains rarely run beyond four or five links and why the pyramid of energy is always upright.
Energy enters an ecosystem in one place only — sunlight fixed by producers — and then moves in one direction, from producers to herbivores to carnivores and finally to decomposers, leaking away as heat at every step. Two numbers describe the whole system. At the entry point, green plants convert only about one per cent of the sunlight falling on their leaves into chemical energy. At every step after that, roughly ten per cent of the energy in one trophic level is passed up to the next. Nutrients, unlike energy, are recycled; energy is not, which is why an ecosystem needs a continuous solar input to keep running.
Statement-based ecology questions in UPPSC are usually direct quotations from NCERT, so the safest test is whether the sentence sounds like the textbook. Both sentences here do. The one place a well-read candidate can talk themselves out of the right answer is the one per cent figure, because senior-secondary biology quotes plants capturing two to ten per cent of incoming radiation. Those are not contradictory claims — they use different denominators. Two to ten per cent is of photosynthetically active radiation, the narrow band of light plants can actually use; about one per cent is of the total solar energy striking the leaf. UPPSC is quoting the second, so statement 1 stands. Once that is settled, the ten per cent law is uncontroversial, and (b) follows.
- Standard NCERT statement: green plants in a terrestrial ecosystem capture about one per cent of the sunlight energy falling on their leaves and convert it into food energy
- Lindeman's ten per cent law, set out by Raymond Lindeman in 1942: on average only about ten per cent of the energy at one trophic level is transferred to the next, the remaining ninety per cent being lost mainly as heat in respiration and in unassimilated matter
- Because energy shrinks about tenfold at each step, food chains seldom exceed four or five trophic levels and the pyramid of energy is always upright, even where pyramids of number or biomass can be inverted
- Energy flow through an ecosystem is unidirectional and non-cyclic, whereas nutrients circulate through biogeochemical cycles and are used again
- Gross primary productivity minus the energy respired by the plants equals net primary productivity — the food energy actually available to herbivores
- Sunlight falling on leaves — 100 per cent of the incident solar energy
- Producers (green plants) — capture about 1 per cent as food energy (statement 1)
- Herbivores, primary consumers — about 10 per cent of what producers hold (statement 2)
- Carnivores, secondary consumers — again about 10 per cent
- Top carnivores — again about 10 per cent; the chain runs out of energy here
About 1 per cent gets in, then about 10 per cent moves up at each step. Both statements describe this diagram, so the answer is (b), Both 1 and 2.
- Mixing the 'about one per cent of incident sunlight' figure with the 'two to ten per cent of photosynthetically active radiation' figure and marking statement 1 false — the two use different denominators
- Applying the ten per cent law to pyramids of number or biomass; it is a statement about energy transfer
- Assuming energy is recycled the way nutrients are — energy flow is strictly one-way and must be replenished by sunlight
UPPSC reproduces the NCERT sentence almost verbatim and asks whether it is correct, sometimes as a single-line question on what happens to energy between trophic levels; UPSC asks the same physics sideways — pesticide concentration along a food chain, the productivity ranking of ecosystems, or the correct sequence of a food chain.
Which one of the following is the correct sequence of ecosystems in the order of decreasing productivity?
- (a) Oceans, lakes, grasslands, mangroves
- (b) Mangroves, oceans, grasslands, lakes
- (c) Mangroves, grasslands, lakes, oceans
- (d) Oceans, mangroves, lakes, grasslands
Answer(c) Mangroves, grasslands, lakes, oceans
The productivity side of statement 1 — how much energy different ecosystems actually fix. Ranking mangroves, grasslands, lakes and oceans by productivity is the same question of how much sunlight producers capture, asked comparatively.
A pesticide which is a chlorinated hydrocarbon is sprayed on a food crop. The food chain is: Food crop – Rat – Snake – Hawk. In this food chain, the highest concentration of the pesticide would accumulate in which one of the following?
- (a) Food crop
- (b) Rat
- (c) Snake
- (d) Hawk
Answer(d) Hawk
The mirror image of statement 2. Energy falls by about ninety per cent at each trophic step while a fat-soluble pesticide concentrates at each step, which is why the hawk at the top of the chain carries the highest residue. Learning the two together locks in what does and does not increase up a food chain.
The amount of energy during transfer from one tropic level to another in an ecosystem
- (a) Increases
- (b) Decreases
- (c) Remains constant
- (d) May increase or decrease
Answer(b) Decreases
Exactly statement 2, asked as a one-line question: energy decreases as it passes from one trophic level to the next. That decrease is the ten per cent law.
What is true about ecosystem ?
- (a) Primary consumers are least dependent upon producer
- (b) Primary consumers are out-number producers
- (c) Producers are more than primary consumers
- (d) Secondary consumers are the largest and most powerful
Answer(c) Producers are more than primary consumers
The consequence of the same rule. Because only about a tenth of the energy moves up, producers must vastly outnumber and outweigh primary consumers — the reason 'producers are more than primary consumers' is the true statement there.
- practice — not a real PYQ
The 'ten per cent law' of energy transfer between successive trophic levels in a food chain was proposed by
- (a)Raymond Lindeman
- (b)Arthur Tansley
- (c)Eugene Odum
- (d)Ernst Haeckel
Answer(a) Raymond Lindeman — he set out the law in 1942; Tansley coined the term 'ecosystem' and Haeckel coined 'ecology'.
- practice — not a real PYQ
In an ecosystem, the flow of energy is
- (a)Unidirectional
- (b)Bidirectional
- (c)Cyclic
- (d)Multidirectional
Answer(a) Unidirectional — energy moves from the sun to producers to consumers and is lost as heat; only nutrients are recycled.